How difficult would it be to add a backbone to the Hurricane ladder frame. One of the things that is important to me in a chassis is torsional rigidity. I want a chassis that resists twisting around hard corners. This helps reduce large amounts of stress in the fiberglass body.
Looking at the pictures, it looks as if a backbone could be fabricated underneath the tunnel. Does anyone who has built one see a reason why a backbone could not be fabricated there?
Anyone think that Hurricane might be willing to add one during the chassis build?
Thanks!!
This thread fits with a question I've had recently, and that is the Hurricane's handling and ride characteristics. Based on all the discussion I've heard FFR's calling card is their frame design, which supposedly make for exceptional handling. I'd much rather build a Hurricane for a lot of other reasons, but can't help but wonder if I'd be disappointed in the handling performance. I'm not looking to build a race car, but tend to enjoy spirited driving on the road and would hate to kick myself for being disappointed in the handling and/or ride department. I intend to use a fixed rear either way, and wonder how much of a difference there'd be between handling of a Hurricane and a FFR set-up the same way?
Hey guys,
I think Mike may be out of town on vaction, so hang tight for the moment.
You can read the article on ERA's site about their frame design. Hurricanes frame is very similar, including the 'X" brace.
Larry
http://www.erareplicas.com/misc/stress/deslogic.htm
I've wondered about the same frame modification.
I can tell you this about mine, I had mine on jackstands with the body/doors/hood on. I took one jackstand out from under the passengers front wheel, leaving that side hanging in w/o support. It didn't change the fit of the doors or hood at all. Don't know if that helps or not but I thought it was cool!
Dan, As far as side impact protection (or front or rear or top for that matter), none of these kinds of cars offer much protection. One thing that does helps us is the cars are so visible and attract so much attention that accidents are rare with them. You'll see... you'll pull into a gas station and poof, instant car show! Twenty people will be looking at your car.
Accidents AROUND US aren't rare as people drive into one another because they're looking at our cars and not paying attention to what they're doing! I've seen it happen!
Larry
I've started to answer here several times but I keep deleting my $0.02. But now that Larry has chimed in, let me support his statements;
Structural design is an incredible science. I think that the manufacturers that tout the, "Our frame is better than their frame because..." are taking bits & pieces of the science and probable/possible loading scenarios and using whatever data fits their particular side to enhance their argument.
I too was once convinced by the FFR hoopla that theirs was the only real choice. Then I made a mistake one day. I put my older brand-X Cobra replica on my two-post lift and stuck it 6' into the air. Once it was up I saw that I had missed the frame with one of the pads. Like Larry, I was amazed to find that the frame had Z-E-R-O deflection. Now this is an older design rectangular frame with NO corner gusseting or reinforcements of any kind. Quite literally the simplest ladder frame I have ever seen.
I then stuck both my Corvettes on the lift and careful raised them with three arms. Their frames both deflected. Hmm... So my basic ladder is stiffer than my so-called great handling Corvettes. Then I did the same with my 3/4 ton truck. It tweaked quite a bit.
When I went shopping for Cobra #2 I saw a raw Hurricane frame. I was amazed to see gusseting and bracing where I don't think it would ever deflect even in an IRL race. So my older Brand-X replica with the simple un-reinforced ladder frame is stiffer than any of my other cars. The Hurricane frame is gusseted & braced quite well over my other ladder frame. That was all I needed to see.
I don't buy into the manufacturer
My question was not simply based on the FFR or other manufacturer claims that a backbone is stronger. I am a structural engineer and have a pretty good understanding of the science behind strengths. The basic concept is that a section, be it a steel beam or a car frame, gains its strength from the vertical dimension of its section. The section modulus determines the strength of the section and the major factor of the section modulus is the vertical component. That is why most steel beams are of the "I" variety with very little steel in the cross section. A small amount of steel, but laid in a vertical section resists bending quite well.
I understand that when your car is on the lift, but only partially supported that no deflection is noticeable. When the car is under power though, and tearing through a corner at 20-30mph or more, the forces are not only that of gravity. The forces are multiplied several times due to many factors including speed, front/rear weight ratio and others. In other words, while the frame may only be deflecting a quarter of an inch when hanging from a lift, it could be 3/4 to an inch when cornereing depending on the actual forces involved.
Adding a backbone to the chassis gives it that vertical component that will resist bending much more. The stiffness in action is what i'm concerned with.
I do like the looks and the kit components of this Hurricane car. I'm not sure how soon I'll be buying, but it will probably be within the next 8 months to a year. I just want to know if this is a possibility for the chassis. I'm not tryong to start any brand wars! :)
Sorry about the technical diatribe as well. If any of you guys know or are engineers, then you know we like to talk about this stuff! ;D
Well you just blew ME out of the water!!! ;D
Quite frankly, that's why I chimed in then deleted my post the first three times! I didn't want to sound like I knew too much about what I was talking about, since I don't!
In looking at my setup, I'm thinking that the center section of flooring could be replaced (or reinforced) with a rigid member if you wanted to do that without a lot of fuss. The center tunnel is fiberglass and duplicating it in sheet steel and beading it for rigidity wouldn't be difficult at all for a do-it-yourselfer. But wether the HMS folks would do it, well, that's out of my league.
Tom
Sorry! ;)
I'm not here to blow anybody out of the water! I'm certainly not looking to put down the Hurricane car, as I think the chassis and body both look well designed and thought out. Thanks for the info. Looking at some of the build pictures I have seen, it looks as if the steel between the driver compartment and the engine compartment is a bolt in item. Is this correct? If so, I think it might be problematic to add the backbone. Also, I was thinking more along the lines of fitting a tube frame underneath the tunnel, and leaving it intact. If I was to cut out a section of the fiberglass shell that large, it might not want to hold together too well. I have to admit, I'm fairly new to the Hurricane, so I don't know how it is all put together. Do you think there is enough room under the fiberglass tunnel to fit a steel tube frame, or would there be no room left for the driveline?
Thanks for the help!!
JayHawk & Tom:
I know nil about frame design, and don't own a Hurricane or any other brand Roadster at the moment, so this has been a very informative dialog. While rigidity obviously has a lot to do with a car's handling abilities, I wonder what other aspects of frame design are primary contributors to performance in this area. Sounds like frame design is a bit art, or at least trial and error, but what about the Hurricane's frame design leads you to believe it will deliver exceptional handling? I wish there were more Hurricane's on the road so I could experience it first hand. Hopefully later this summer.
Dan
Interesting discussion.
I'm not an engineer but I do have road racing experience. In my opnion, even if using racing tires heated to temp, the loss of traction will be reached before the limits of the frame came into play. Only a world class driver (of which I am NOT by any means) would be able to extract the difference between a purpose built comp frame and most of the current roadster replicas. I've driven an SPF and it feels as solid as my brothers FFR and the SPF frame didn't impress me. (nice car though, don't get me wrong!)
As for adding a backbone, I don't think there is enough room as you approach the back of the car. The tunnel gets small at the rear bulkhead. Add a driveshaft to the space and there isn't much left. Maybe it could be done if kept small but I doubt it.
If the car is going to be used in competition though, a cage will have to be added anyway making the backbone un-needed.
All my opnion of course.
Larry
I'm an engineer also (and an original but former Jayhawker) so it's in our genes and we just can't help explore these questions.
A friend of mine who used to manufacture kits schooled me on some of the things to look for in a car with a ladder frame to determine how much flex there is. He maintained that a good indicator is to put your finger in the joint between the door and the body while doing some hard cornering. If the chassis does have any significant flex, that gap will open and close depending on the forces. He said that on some kits he's been in there is enough flex in that gap to pinch your finger!
Don't get me wrong on my abilities. I am not a chassis/frame designer. I design building structures by trade. When it comes to geometry/roll center, and that type of thing, I really only know what I've read in the Chassis Engineering book by Herb Adams. It is actually a really good book. The structural abilities of the frame are basically the same as any structure though. Loads are loads as well. The only difference is what is making the loads and where they are being applied. I guess what I'm saying is that I'm not an expert on chassis design.
Thanks for the input guys, this has been a great topic with some excellent input. Oh, by the way, great to see another Jayhawker!!
LMH
As far as losing traction before the frame will flex goes, it will depend on the steepness of the curve and probably some other factors as well. What it boils down to is how much force is applied to a corner of the vehicle and at what angle. The force breaks down into a vertical and horizontal component. The horizontal will want to push the car sideways possibly breaking traction while the vertical will push down on the corner actually adding to the friction resisitance making it less likely that the tire will break loose. The down force on that corner also has an equal and opposite up force at the opposite corner. This it what causes the frame to flex.
You are absolutly correct that adding a cage ends any need for a backbone as well. The cage will stiffen the chassis much more than a backbone will. I'm not looking for an all out competition racer, but something that is a great street car that will also perform well on the track days I'd like to attend every now and again.
Thanks!
Herb Adams makes a lot of sense. But don't forget that he was firmly entrenched in the soft spring / huge antisway bar camp in the 70's when all the other engineers were in the stiff-spring / anemic bar camp. A big reason the Trans Am rode softer than the Camaro. He even put out a Cobra replica in the late 80's / early 90's (IIRC).
Most of the Cobras you'll find will be engineered for springs sticking things to the ground without swaybars.. I'm pretty sure that someone here adapted (fairly easily) a sway bar to the Hurricane. For some reason I'm thinking it was a Ford Escort bar, but my memory could be off. In any event, I'm all behind Herb Adams' handling theory as long as you're on a smooth racetrack. But on uneven pavement, I think the springs should be the primary leveling force.
Gee, was this even part of this original thread or do I have to go to my room for hijacking??? ;)
Oh, hijack away! No problem with me. It all ties in to the handling anyway. I did enjoy the Adams book, but it has been a few years sisnce I read it. I need to get it out again and go through it.
I believe you are right about the differing styles though. These cars are most often designed for non-sway bar stiffness.
Great conversation though! :)
I read his book too a while back. I'm kind of in agreement with him about softer springs and bigger anti-roll bars. In a street car, hard springs are really hard on kindeys, my butt and a few other body parts as well! My Hurricane is sprung hard IMO and I may change that as I experiment with it.
That will all come later though. Right now, I'm figuring out which direction to go with the engine.
Larry